摘要
最近高压单晶X射线衍射实验明确显示正交晶系(Pbnm或Pnma)(钙钛矿)ABO3的高压压缩行为完全由晶体框架结构(framework)中的BO6八面体和AO12多面体的相对压缩性所确定,即对一些钙钛矿晶体,BO6较AO12多面体更易压缩时,压力导致晶体结构向高对称性结构演变;对另外一些钙钛矿,BO6较AO12多面体更不易压缩时,压力导致晶体结构向低对称性结构演变.基于价键匹配关系假设,建立多面体压缩率之间关系:βB/βA=MA/MB,由价键参数和常压晶体结构数据计算参量Mi,可以估计BO6和AO12多面体的相对压缩率,从而正确地预言高压压缩行为.文章最后引入高温高压钙钛矿倾斜相变的一般性规律,即MA/MB>1时,相变温度和高压关系为dTc/dP<0,当MA/MB<1时,dTc/dP>0.
Recent high-pressure single-crystal X ray diffraction experiments show that when the octahedral BO6 group is more compressible than the AO12 polyhedra in certain ABO3 （Pbnm or Pnma） perovskites （βB 〉βA）, then the BO6 octahedra becomes less tilted and the structure evolves towards a high-symmetry configuration as the pressure increases. In other perovskites the BO6 octahedra are less compressible than the AO12 polyhedra （βB 〈 βA ）, and the structure then becomes more tilted with increasing pressure. A new model, based on the assumption of bond-valence matching in both the A and the B sites of a perovskite, provides a simple and universal rule that describes the dominant mechanism governing pressure-induced octahedral tilts and distortions in both polyhedral AO12 and BO6. By the relation βBβA = MA/MB, where Mi is a site parameter calculated by using bond-valence valence parameters and crystal structural data under ambient conditions, the high-pressure behavior can be correctly predicted . A general law that predicts the variation in transition temperature of tih transitions in perovskite has been deduced ： in perovskites with MA/MB 〉 1, the temperature Tc of zone-boundary phase transitions will decrease with increasing pressure, dTc/dP 〈 0, while with MA/MB 〈 1, dTc/dP 〉 0.
出处
《物理》
CAS
北大核心
2006年第6期461-465,共5页
Physics
基金
美国NSFgrant EAR-0105864和EAR-0408460资助项目
关键词
钙钛矿
高压
高温
八面体倾斜
价键模型
相变
perovskite, high pressure, high temperature, bond-valance model, phase transition
